Unsupported XML version

What it means

XML has had exactly two versions in its life. 1.0 is what essentially everything reads and writes; 1.1 exists mainly to permit control characters and additional line-ending forms, and support for it is thin enough that choosing it is usually a mistake.

There is no 2.0, and there never has been. A version number above 1.1 is always a typo, a placeholder, or a confusion with the version of something else — a schema, a feed format, an API.

What usually causes it

• Confusing the XML version with the document format's version: RSS 2.0, SOAP 1.2, UBL 2.1.
• A template variable for the payload version leaking into the declaration.
• A copy-paste that grabbed a version number from the wrong line.

How to fix it

• Use version="1.0". It is the right answer for nearly every document.
• Reach for 1.1 only if you specifically need its relaxed character rules, and check every consumer supports it first.
• Keep the format's own version where it belongs — the root element's attribute, not the declaration.

The same error elsewhere

Different parsers, same defect. If you arrived with one of these messages, you are in the right place.

• Xerces (Java)

  XML version "2.5" is not supported, only XML 1.0 is supported.

• expat (Python)

  XML declaration not well-formed: line 1, column 20

• .NET System.Xml

  Version number must start with '1.'